drm/omap: remove unnecessary pitch round-up

omapdrm checks if the pixel stride is divisible by 8. This is meant to
ensure that the byte stride is 32, which is required by SGX.

The check is not correct, as it checks for pixels, not bytes, and thus
needlessly increases the stride for, e.g., NV12.

Also, SGX driver is not supported in the mainline, and the TI's SGX
driver nowadays does the memory allocation itself and doesn't rely on
omapdrm to figure out the correct pitch.

So we can just remove the whole roundup.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>